The present invention relates to analytical instruments and particularly instruments using an inert carrier gas in fusion or combustion furnaces for determining low concentrations of one analyte in the presence of high concentrations of another analyte.
In analytical instruments, typically a sample to be analyzed, such as a solid, liquid, or gas sample, is combusted in a furnace and subsequently the byproducts of combustion are swept by an inert gas, such as helium, into an oxidizer which converts, for example, carbon monoxide from the furnace into carbon dioxide for subsequent infrared detection. In order to analyze substances other than carbon dioxide (representative of the oxygen content of a sample), it is necessary to scrub and remove the carbon dioxide from the carrier stream so that the remaining analytes, such as nitrogen, can be measured by a subsequent detector, such as a thermal conductivity cell located downstream of the scrubber. With such systems, however, the scrubbing of carbon dioxide or other gas from the carrier stream reduces the pressure and flow rate of the carrier gas and remaining analytes as it approaches subsequent detectors for determining the remaining analyte. These perturbations in pressure and resulting flow rate result in inaccurate measurements of analytes subsequent to the scrubbing operation. In an oxygen/nitrogen analyzer, for example, the nitrogen, which is measured downstream of the scrubber with a thermal conductivity cell, is adversely affected and can result in readings which have a significant error. The error, for example, could be as high as 35 ppm (parts per million) for a nitrogen sample having a 5 ppm concentration in a sample having a high oxygen concentration of 50,000 ppm.
Although attempts have been made to overcome this problem using a split stream of analytes, such systems require a more complex and expensive flow system, and it is difficult to maintain the ratio of analytes split into two flow paths constant. Further, the analytical time is increased significantly as in the carrier gas consumption. Also the resultant accuracy, although improved, remains erroneous when measuring a low concentration analyte in combination with a high concentration analyte.
As a result, there exists a need for a relatively economical analysis system in which a relatively small concentration of an analyte in the presence of a higher concentration of a second analyte can be accurately determined without interference due to perturbations in the pressure or flow of the carrier-gas analyte stream.
The system of the present invention overcomes the problems of prior art analyzers by providing a compensation system for providing a makeup flow of carrier gas to maintain the pressure and/or flow rate of the stream of analyte and inert gas to a detector constant such that a accurate determination of a low level of an analyte can be accurately determined. In one embodiment of the invention, a carrier gas is introduced through a controlled valve responsive to a detected pressure in the gas flow stream between a scrubber and a subsequent detector for maintaining the pressure constant during an analysis. In another embodiment of the invention, a flow transducer is positioned in the gas flow path between the scrubber and detector and coupled to a flow control valve to introduce carrier gas as a function of detected gas flow such that the flow rate of gas into the detector is maintained constant. In either embodiment, a relatively low level of analyte, such as nitrogen, in the presence of a relatively high concentration of another analyte, such as oxygen, can be accurately determined utilizing a flow system which has a minimal number of components. Such a system provides greatly improved accuracy for the economical measurement of a relatively low level of analyte in the presence of a second analyte having a higher concentration.